Electrical power lines transmit electrical noise to equipment powered by the power lines. One category of such noise is common mode noise. Common mode noise is a common mode voltage (CMV) or voltage that is present on both leads of an analog/digital input with respect to a ground. Common mode voltage is distinguished from a normal mode voltage (NMV), in that a normal mode voltage is a voltage that exists between the two leads of an input. The most common source of common mode noise is the difference in potential between two physically remote grounds. In poorly grounded power systems for example, this potential difference can raise both the power and neutral power lines supplying the electrical equipment 110 up to about 30 volts above the power supply ground. Even in well grounded power systems, this voltage differential may be in the neighborhood of 1 to 2 volts.
While ground potential differences are the major source of common mode noise, they are not the only source. Among the other causes of common mode noise are improperly grounded signals from other equipment tied to the power lines and radio frequency signals from sources such as microwave induction heaters and motor control relays. All the described types of noise signals result in ground current which could disrupt the operation of the equipment powered by the power supply. For instance, a computer ground current can cause damage to the computer circuitry or loss of data due to power supply voltage perturbations when power on reset of the power supply occurs.
Proper design and testing in the laboratory usually eliminates most power source problems. However, laboratory testing may not always identify common mode noise conditions encountered in the field that result in operational problems or damage to computer equipment. The common mode noise conditions encountered in the field can be continual or intermittent noise signals. Furthermore, existing systems are not always able to produce sufficient energy to simulate significant CM or ground bounce such as may be experienced with larger sysplex computer systems. In any case, it would be desirable to be able to test for common mode failure conditions, particularly those failure conditions encountered in the field, to eliminate susceptibility to the noise.
Therefore, what is desired in the art is the ability to evaluate electrical equipment for common mode noise susceptibility simulating common-mode voltage conditions encountered in the field under laboratory conditions to diagnose power failures and modify the systems to overcome those failures.